Compositor-backed canvas contexts keep their transports tied to a single
Compositor connection. When that connection dies, the 2D backing
storage and WebGL GL objects disappear with it, but WebContent does not
surface the loss to canvas contexts or create fresh host contexts after
reconnect.
Track compositor loss through the WebContent connection, mark WebGL
contexts lost, dispatch the standard context events, and rebuild the
remote proxy when the page opts into restoration. For 2D canvas, queue
the canvas context loss steps, discard the dead backing storage, and
create new storage before firing contextrestored.
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